1. Field of the Invention
The present invention relates to a driving circuit and a method thereof, and more particularly to a light-emitting device driving circuit and a method thereof.
2. Description of Related Art
FIG. 1 shows a conventional buck type light-emitting diode (LED) driving circuit which senses source current of a metal oxide semiconductor (MOS) transistor and a coupling method of the buck type light-emitting diode driving circuit. The light-emitting diode (LED) driving circuit consists of a timing-generating circuit 202, an SR latch 204 positively triggered, an AND gate 206, a switch 208 implemented by an N-type metal oxide semiconductor (NMOS), and a current-sensing circuit 210 so as to drive an LED string 212 formed by a plurality of LEDs. Certainly, the switch 208 may also implemented by P-type metal oxide semiconductors (PMOSs), bipolar junction transistors (BJTs) or other types of transistors. In FIG. 1, a terminal of the LED string 212 is coupled to a direct current (DC) type supply voltage VIN and the cathode of a diode 216 via an inductor 214. The other terminal of the LED string 212 is coupled to the anode of the diode 216.
The timing-generating circuit 202 is used for generating a turning-on control signal TS and changing a timing sequence of the turning-on control signal TS according to a timing control signal EXCS. The current-sensing circuit 210 determines whether or not to generate a turning-off control signal RS according to a value of a conducting-current IDRAIN of the switch 208. Thus, the SR latch 204 can receive from a set terminal S and a reset terminal R the turning-on control signal TS and the turning-off control signal RS respectively, and change an output of an output terminal Q according to the two signals. Consequently, the SR latch 204 controls whether the switch 208 is turned on or off via the AND gate 206 so as to control magnitude of an inductor current IL. As to DIM shown in FIG. 1, it represents a dimming signal in the form of pulse width modulation (PWM), and the dimming signal DIM is used for adjusting luminance of a light source emitted from the LED string 212 and determining whether or not to turn off the switch 208.
FIG. 2 is an oscillogram of the inductor current IL and the conducting-current IDRAIN of the circuit shown in FIG. 1. Referring to both FIGS. 1 and 2, it is learned from the two drawings when the conducting-current IDRAIN reaches a predetermined peak value IDMAX, the switch 208 is turned off and will not be turned on again until after a period of time T. Such operation causes the inductor current IL to fluctuate between a maximum current value IMAX and a minimum current value IMIN so as to stabilize the luminance of the LED string 212. When the switch 208 is turned off, the current-sensing circuit 210 cannot sense current. Therefore, the conventional LED driving circuit needs the timing-generating circuit 202 to regularly provide the set signal required by the SR latch 204 so as to further control a discharging time of the inductor 214.
However, since the timing-generating circuit 202 depends on the timing control signal EXCS to change the timing sequence of the turning-on control signal TS, when variation in the inductor current IL exceeds a range defined by the maximum current value IMAX and the minimum current value IMIN and requires the timing sequence of the turning-on control signal TS to be changed, the user must modify a circuit which provides the timing control signal EXCS in order to alter the timing control signal EXCS. Otherwise, an inductor value of the inductor 214 or a number of the LEDs in the LED string 212 needs to be changed to alter the timing control signal EXCS. Such situations cause inconvenience to the user. Further, if the LED driving circuit is manufactured as a circuit chip, additional pins are also required to couple with an external circuit which provides the timing control signal EXCS, which in turn also causes disturbance while designing.